In the most common form of silica chromatography, more polar molecules stick to the stationary phase. Silica is just sand, and the polar silanol groups (-Si-OH) interact with the polar parts of the molecule. You can "reverse" the properties of silica by converting the silanols into something nonpolar.
ODS-Cl was the first reagent commonly used to do this - it converts the -Si-OH into -Si-O-Si-O-(CH2)15CH3. This means that polar molecules are effectively indifferent to this modified silica and will flow right past, while greasy things will stick like crazy. This is often a useful thing, and "reversed phase" chromatography is ubiquitous in HPLC.
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